The Sad Story of the Suddenly Sticking Solenoid

The scenario starts without warning. You spin the key to start your vintage Ford, and just like that, it happens. You release the key, which goes back to the “Run” position just like always, but the starter does not disengage. It stays engaged, as if still trying to start the engine.

But the engine is already started, and now the engine is driving the starter, and driving it much faster than it was designed to run. Something is going to overheat and either break or burn. And it’s going to happen soon. If it’s not stopped quickly, the wiring will overheat, melt the insulation, and possibly cause a fire. You’d better hope you’re not leaking any gas.

It’s the story of the Suddenly Sticking Solenoid, and the Diagnostic Detectives are on the case. A couple of years ago, we watched a meticulously restored ’68 GT500KR suffer this fate. The owner went to start it on a weak battery, and when he released the key from “Start,” the circuit stayed engaged, beginning a severe overheat situation. He quickly removed a battery terminal and headed off bigger problems.

Another similar episode with worse results involved a big-block car with a dragging starter. It was coming off a highway run through the summer heat. Pulling off the interstate and briefly shutting off the engine, all seemed well. But while restarting the engine a couple of minutes later, the starter acted weak, unable to turn the engine over fast enough to start it. Releasing the key, the starter failed to disengage. Smoke appeared from under the hood, then stopped. Raising the hood, the heavy gauge wire from the positive battery cable had become so hot, it melted away from the lead terminal, acting as a functional fuse. The starter had failed (warning sign — it cranked slower when hot), drawing maximum current, enough to cause the solenoid to stick closed, and setting the meltdown in motion.

Just about every Ford owner will deal with a bad solenoid at least once, and it may or may not involve a short circuit and/or a near meltdown. The Diagnostic Detectives have investigated and uncovered some facts that can help you keep your classic Mustang (or other Ford) on the road without a meltdown.

HOW DOES IT WORK?

The solenoid, or starter relay as it’s called in Ford lit, is comprised of two separate circuits. When you turn the key to “Start,” a low current circuit is energized, magnetizing a metal shaft. On one end of that shaft is a large metal disc. As the shaft moves, it pulls the disc into contact with the two high-current terminals, one from the battery, and the other to which the starter cable is attached, completing the circuit and letting lots of power flow to the starter. When the key is released to the “Run” position, the shaft is no longer magnetized, and a spring pushes it away from the two high-current terminals.

MALFUNCTIONS

Ever wonder what’s in a starter relay? The main part is the big disc that’s magnetically pulled down to contact both terminals. That closes the circuit between the battery and the starter.

Wear is clearly visible on the left terminal, but the purple area on the right contact is discolored from heat. That’s where the arc occurred that caused the relay to stick.

When the key is released from “Run,” it’s up to the spring’s tension to push the big metal disc away from the high-current terminals and break the connection. If the ignition switch or its wiring fails, the starter relay will stay engaged. But this is rare.

Far more common is the big metal disc becoming attached to the terminals due to electrical arcing — mini welds caused by excessive current draw by the starter, inferior metals used in the relay, heavy wear, or a combination of all.

As part of the investigation, the Diagnostic Detectives opened up the actual starter relay that had failed in one of the aforementioned cases. Though it had returned to the open position, the wear was easy to see, as was a discolored burn spot. Once the disc and terminals had cooled, it released and may even have become functional again. In a pinch, the terminals could have been rotated 90 degrees to expose a fresh surface. But we weren’t about to take that chance.

PREVENTION AND CORRECTION

Dirty Starter Relay.JPG

“I just bang on them with my hand,” says Mustang Magazine Q&A columnist and R&A Motorsports President Jeff Yergovich. That jars the disc loose. Quickly removing a battery cable also opens the circuit. Mounting a manual on-off switch near the battery is a good idea too. And a fire extinguisher should be a must in any vehicle, especially an older collector car. Once the relay has stuck, it should be replaced. Don’t give it another chance to burn up your wiring, or worse.

Jeff told us that the only relays he has ever seen stick are cheap aftermarket units. The Diagnostic Detectives also spoke with a Ford technician who told us that Ford has upgraded their metallurgy on starter relays over the years to prevent this problem, so this is an instance where insisting on the Ford brand part is a good idea. For what it’s worth, the relay we opened up for this article was stamped as being made in the USA, but it did not have a Ford part number.

Discussions on Internet forums also mention that a weak battery seems to be a contributing factor, so keeping it charged up with a smart charger like a Battery Tender is a good idea.

ADDITIONAL INFO

PREVENTION TIPS

• Keep your battery at a full charge.
• Use only a Ford brand starter relay.
• Install an emergency electrical cut-off switch on your battery.
• Replace a weak starter before it fails completely.
• Carry a fire extinguisher in your car at all times.
• A stuck relay can often be unstuck by hitting it.